Ablation methods within tissue volumes are additionally supported these days by imaging systems, e.g. computed tomography scanners, magnetic resonance imaging scanners, ultrasound equipment or imaging systems based on similar imaging methods. Using the image data obtained therewith, ablation instruments can be navigated to a certain target within a tissue volume.
There are different ablation methods which all have in common that an instrument is positioned in the target tissue volume in order to destroy the tissue at the location: In the case of so-called cryoablation, this is performed by icing. The advantage of this method is that the icing zone is very recognizable in a computed tomography image. A very new method utilizes the effect of electroporation. Here, the cell membranes are changed by applying a very high, pulsed DC voltage and this leads to the death of the cell. The most frequently utilized method is RF (radiofrequency) ablation, in which radiofrequency waves are used to place thermal energy into the target tissue volume—that is to say the tumor is “cooked”. In an exemplary manner, this method will be discussed in more detail below.
In the case of RF tumor ablation, a needle-shaped applicator is inserted into a tissue and pushed to the location of a tumor under monitoring on the basis of the image data from computed tomography. Once the applicator is in the target area, thermal energy is produced by microwaves and leads to the destruction of the tumor tissue.
There are different types of RF probes, with, in principle, it being possible to distinguish between needle-shaped probes and umbrella probes. In the case of umbrella probes, a number of individual antennas are deployed after positioning. They thus attain a larger ablation volume than the needle probes. In the case of needle-shaped probes, a plurality of probes (“cluster”) are often used simultaneously in order to attain a certain volume.
During an ablation process, a temperature sensor installed in the respective probe or an impedance measurement determines when the end of the ablation has approximately been reached: In the case of an impedance measurement, an increase in the impedance, the so-called “roll-off”, can be determined toward the end of the ablation. The roll-off is created when the corresponding tissue has been ablated and the conductivity decreases. It is usual to go through approximately two roll-offs before the ablation is finished. There can be an analogous process in the case of the temperature measurement too, with a temperature threshold being exceeded signifying the end of the ablation in this case.
In addition to the type of applicator, the extent of the ablated area depends inter alia on the application duration and the applicator power. The corresponding parameters are set in advance by a user on the ablation instrument as a function of different basic information. The basic information takes account of, for example, the size and extent of the respective tissue to be ablated, the type of tissue, vessels lying in the vicinity which could dissipate the heat and particularly sensitive zones such as nerves which shall not be adversely affected.
The product information by applicator producers often contains tables which reproduce the thermal expansion as a function of the instrument setting. These tables were determined on dead tissue and cannot go into the individual ablation situation. It is for this reason that they are not sufficiently accurate.
Hence, the most common method for setting the parameters currently is that a user works on the basis of his own experience. Naturally this still results in a high uncertainty in tissue ablations and, additionally, decisively depends on the experience of the respective user. This holds true in particular if the tissue volume to be ablated exceeds a diameter of 5 cm. In that case it can be expected that for example not all tumor cells of an affected tissue are acquired by the ablation. Therefore, in general, only tumors with a diameter of up to at most 5 cm are currently ablated.
There is a further risk that such an ablation can also adversely affect sensitive structures in the vicinity of the ablation area, e.g. nerves. Moreover, ablation is particularly difficult if a blood vessel is present in the vicinity of the ablation zone, which blood vessel dissipates heat and thus changes the thermal ablation volume. All these problems lead to the fact that ablation processes can to date only be controlled imprecisely.